I can see the reasoning behind deleting closed proposal — if someone proposes the same subject again, they get a fresh start. This isn't completely beneficial though: it leads to repeating the same failed attempts over and over.
We have discussions about proposals, which are far more useful than a list of questions in understanding what may have been good or bad about a past proposal. Unfortunately, when a proposal is deleted, these discussions are deleted as well. So no trace remains of any advice or debate regarding how a subject may or may not be fit for a questions and answers site, what scope would be reasonable, etc.
For example, consider the question How would a Debian-specific stack be different from the existing Unix & Linux site? I think I've answered this very question before (about a prior, now-deleted proposal about Debian), and I'm sure I've answered this questions about other distributions (from memory, Arch Linux and Fedora, but I may be misremembering). But I would have to write the same post yet over again, because all the previous discussions have been deleted.
Please do not delete discussions about deleted proposals. Add a big fat warning if you like, or classify them in a different section, but leave the discussions publicly accessible.